Orthogonal Frequency Division Multiplexing (OFDM) is a known type of radio frequency (RF) modulation specified for use in local area networks (LANs) such as those operating in accordance with the 802.11a LAN industry standard in the 5 GHz frequency band. This band is a non-coordinated, license-free band allocated for high-speed (up to 56 Mb per sec) data transfer.
There is a derivative of the protocol defined in the 802.11a LAN standard which is being proposed for use in the 4.9 GHz band, intended for public safety use only. Its operation and modulation is very similar to that specified for the 802.11a LAN standard, with some added capabilities.
Generally, the LANs being developed in accordance with the 802.11a LAN standard comprise Remote Units (RUs) and one or more Access Points (APs). An AP and a RU may in practice be communication terminals formed of similar hardware, including a 4.9-5.0 GHz transceiver with a powerful signal processing capability. The RU may in practice be a mobile terminal. The AP may in practice be a fixed terminal physically connected to a static physical network, and communicated data may be transferred to and from the network by the AP.
It is desirable for the distance of a RU to an AP to be known to the AP. A known system for measuring the location of a RU relative to an AP in the 2.4 GHz band, using Bluetooth Frequency Hopping (FH) modulation, includes a transponder at the RU. The transponder includes a dedicated chip. This system requires special hardware and software additions at each RU thus making the RU a non-standard product. Moreover, distance measuring with FH modulation requires the use of a loop-back procedure. FH modulation is basically different to and incompatible with OFDM modulation.